How they work

Rapid HIV tests are single-use ELISAs that contain all the necessary reagents and give results in less than 30 minutes. They work by capturing antibodies (or antigen) on a solid surface and then attaching molecules to them that allow detection by the naked eye.

Test designs are based on three formats.

Most of the devices used in the UK (including Determine , OraQuick and VIKIA) are immuno-chromatography (lateral-flow) devices. They are the simplest to use. The specimen is absorbed into a pad where it immediately combines with a signal reagent. Results can usually be read after a specified period of time (often around 20 minutes) Many lateral flow tests have limitations on how long one should wait before reading them: if you wait too long to read the results the assay will need to be repeated.  All tests include either a procedural control line or a sample adequacy control line that is used to validate the test result. Appearance of two lines, therefore, indicates a positive result, while a valid negative test produces only the control line. If only the test line appears, or if no lines appear, it is invalid and must be repeated.

Other tests, including INSTI, are immunoconcentration (flow-through) devices. The device captures the HIV antibodies in a specimen flowing through a porous membrane. Once the antibodies are absorbed into the membrane, the membrane is developed with a signal reagent and a visible dot appears if the test is positive.

An advantage of this test type is that it is a very rapid test procedure, with results available in as little as a minute. However, the tests need to be performed individually or in small batches and require constant attention.

The first rapid tests were agglutination tests, using different types of particles to produce clumping or settling patterns of the particles when a specimen was positive.